If you would send one sub with 10 drillers to Feld from Garfield, it would immediately lose. The Lieutenant would kill 5 of your drillers, and the 5 shield that Feld will have built up by then will kill the other.

If you would also launch your drillers from Vaccarino, they would also lose 5 drillers to the Lieutenant, 1 to the new shield that will be there at that point, and then the 4 drillers will cancel each other out.

The outcome of the fight is then based on the amount of specialists present, so Feld will win.

My hint is this: the Lieutenant will kill 5 drillers on each sub that approaches Feld. Can you think of a way to combine those 20 drillers into one sub/attack?